Steps to reproduce:

cargo new --lib bitflags-missing-docs
cd bitflags-missing-docs
cargo add bitflags

Replace src/lib.rs with:

bitflags::bitflags! {
    pub struct Flags: u32 {
        /// Hello.
        const HELLO = 1;
    }
}

Run cargo doc --open and search for "hello". It doesn't find anything of that name, and if you go to the Flags page it indeed has no documented constants.

If you change Cargo.toml to use bitflags = "1" and re-run cargo doc, the constant is there as expected: